The reducing action of manganese dioxide upon potassium permanganate
was suspected from the observed increase in the rapidity of the
decomposition of potassium permanganate solutions. But only a
suggestion of such a reaction could be found in the literature.
Thénard,[2] in 1856 states that manganese dioxide may act upon
potassium permanganate either as a reducing agent, in which act the
manganese dioxide is changed to a manganate, or its influence may be
catalytic, causing the evolution of oxygen. Again Mulder[3] in 1858,
ascribed the decomposition of potassium permanganate solutions to the
presence of some potassium manganate.
[2] Comptes Rendues 42, 382
[3] Jahresbericht 1858 p. 581
It could not be ascertained that any investigation of this matter had
ever been undertaken. It was therefore decided to study the question
and it appeared that the most satisfactory evidence could be obtained
by measuring the quantity of oxygen which is evolved when potassium
permanganate and manganese dioxide are brought together.
Description of Apparatus.
The apparatus[4] employed in this work consists of four parts:
“1) a flask, A, in which the reactions were conducted. A melting point
bulb of about 40 cc. capacity, the diameter of its neck being 20 mm.,
was used for this purpose.
[4] This apparatus was used by Allen and described by him. I wish to
acknowledge my indebtedness to him for the illustration and description
given here which is taken by permission word for word from his
dissertation.
[Illustration]
This was closed with a two-hole rubber stopper, through one hole of
which passed:
2) A small glass tube, BB, running up from the flask about 40 cm., and
then bent twice at right angles. One limb of this tube was surrounded
by a small Liebig’s condenser, while the shorter turned down to meet--
3) a Schiff’s azotometer filled with mercury and connected at its top
with the tube above mentioned. In practice it was found desirable to
have a little water on the top of the mercury column.
4) Through the second hole of the rubber stopper closing the flask was
passed a short piece of glass tubing bent obliquely just above the
stopper and tightly connected with a rubber tube about 50 cm. long,
DD. This tube was clamped near its lower end with a Mohr’s pinch-cock,
while the upper was connected with the stem of a small funnel, F.
When an experiment was to be conducted, the azotometer C, was
connected to the tube, B, and tied tightly, a piece of good rubber
tubing being used for the connection. The reservoir of the azotometer,
M, was then raised until the mercury had driven out all the air in C,
and the stop-cock closed. Next the funnel F was filled with water, the
pinch-cock closing D opened and the water allowed to flow down and
drive out all the air in the tube. The latter was then clamped and
thus kept full of water. The apparatus being now ready, the flask A,
containing the substances in the desired quantity, was made fast to B
and D by means of its stopper. The stop-cock of C opened. The flask
A was heated in a water-bath for any desired time, the oxygen being
collected in C as fast as it was given off.
At the end of the experiment, all the gases which remained in A and B
were driven over into C by lowering the reservoir M and opening the
pinch-cock at D. When this was accomplished the stop-cock of C was
closed and the gases brought under atmospheric pressure by bringing the
mercury in C and M to the same level.
It will be seen that in this gas volume was included the air which A
and B contained at the beginning of the experiment.
The oxygen evolved from the contents of the flask was determined thus:
After reading off the total volume of air and oxygen and reducing
to normal conditions, the total volume of oxygen was obtained by
absorption with phosphorus or pyrogallol, and from the residual
nitrogen could be calculated from the volume of air to be deducted
the total volume of gases. The remainder is the volume of oxygen
sought.”
Action of manganese dioxide on potassium permanganate in acid solution.
Three sets of apparatus like the one described were used.
The flask of apparatus No. I contained manganese dioxide and dilute
nitric acid.
The flask of apparatus No. II contained potassium permanganate and
dilute nitric acid.
The flask of apparatus No. III contained potassium permanganate and
dilute nitric acid and manganese dioxide.
The manganese dioxide used in these experiments was precipitated
from a solution of potassium permanganate by a dilute solution of
manganous sulfate in the manner described on page 34 and then washed
and dried at 100°C.
An illustration of the proportions in which the different substances
were brought together in each of the three flasks is to be found in the
following statement of the quantities used in the first experiment.
The variations from these proportions occurring in subsequent
experiments are noted in the table giving the summary of the results.
Usually about 150 m.g. of manganese dioxide were used and the solutions
in the three flasks were brought to the same volume by addition of
distilled water.